Saturday, September 28, 2013

Android Tutorial: Stopping and Restarting an Activity

When your activity is stopped, the Activity object is kept resident in memory and is recalled when the activity resumes. You don’t need to re-initialize components that were created during any of the callback methods leading up to the Resumed state. The system also keeps track of the current state for each View in the layout, so if the user entered text into an EditText widget, that content is retained so you don't need to save and restore it.

Note: Even if the system destroys your activity while it's stopped, it still retains the state of the View objects (such as text in an EditText) in a Bundle (a blob of key-value pairs) and restores them if the user navigates back to the same instance of the activity (the next lesson talks more about using a Bundle to save other state data in case your activity is destroyed and recreated).

It's uncommon that an app needs to use onRestart() to restore the activity's state, so there aren't any guidelines for this method that apply to the general population of apps. However, because your onStop()method should essentially clean up all your activity's resources, you'll need to re-instantiate them when the activity restarts. Yet, you also need to instantiate them when your activity is created for the first time (when there's no existing instance of the activity). For this reason, you should usually use the onStart() callback method as the counterpart to the onStop() method, because the system calls onStart() both when it creates your activity and when it restarts the activity from the stopped state.

Because the user might have been away from your app for a long time before coming back it, the onStart() method is a good place to verify that required system features are enabled.

The tutorial also mentioned onPause() is a good place to release BroadcaseReceiver.